CAN SCREENING CRITERIA FOR DROUGHT RESISTANCE IN NICOTIANA-TABACUM-L BE DERIVED FROM THE POLYPHASIC RISE OF THE CHLOROPHYLL-A FLUORESCENCE TRANSIENT (OJIP)

Four tobacco cultivars (Nicotiana tabacum L.), known to differ with re spect to their drought resistance, were subjected to a slowly intensif ying drought stress (control level: psi(PD) = ca. -0.5 MPa; and severe stress level: psi(PD) = ca. -2.5 MPa), and rewatered under controlled environmental conditions in programmable growth rooms. Drought stress -induced changes in the fast-phase chlorophyll fluorescence induction kinetics during the stress and recovery periods were monitored with a shutterless fluorescence measuring system with a time resolution of 10 mu s (Plant Efficiency Analyser by Hansatech Instr., UK). Although ch lorophyll fluorescence transients are excellent probes for measuring t he effects of stress on the PSII photo-chemistry, in this article we e valuate their practical applicability in screening for drought resista nce. During the entire drought stress and recovery period, the fluores cence transients followed a regular pattern of O-J-I-P--T, with two in termediate inflections J (ca. 2 ms) and I (ca. 20 ms) appearing betwee n the F-o and F-m levels. From the fluorescence data presented, it is clear that the single fluorescence parameter which differed the most i n its response to drought stress between the drought-resistant (GS46 a nd ELSOMA) and drought-sensitive (TL33 and CDL28) cultivars was the F- o value normalized to F-o (t=o), which in confirmation of their drough t resistance, increased dramatically in the former but not in the latt er cultivars. In the interpretation of the results, however, care is e xpressed not to attempt to correlate changes observed in individual tr ansient components with possible physiological causes, since these ind ividual fluorescence parameters are influenced by an array of interact ing physico-chemical and metabolic factors. The use and importance of normalized values, ratios and indices, in terms of the practical appli cation of chlorophyll fluorescence measurements in drought-stress rese arch is discussed.